Package descriptions contain fields for specifying the name of a software
license and the name of the file containing the text of that license. While
package authors may choose any license they like, Cabal provides an
enumeration of a small set of common free and open source software licenses.
This is done so that Hackage can recognise licenses, so that tools can detect
licensing conflicts,
and to deter
license proliferation.

It is recommended that all package authors use the license-file or
license-files fields in their package descriptions. Further information
about these fields can be found in the
Cabal users guide.

Additional resources

The following websites provide information about free and open source
software licenses:

Disclaimer

The descriptions of software licenses provided by this documentation are
intended for informational purposes only and in no way constitute legal
advice. Please read the text of the licenses and consult a lawyer for any
advice regarding software licensing.

The author of a package disclaims any copyright to its source code and
dedicates it to the public domain. This is not a software license. Please
note that it is not possible to dedicate works to the public domain in
every jurisdiction, nor is a work that is in the public domain in one
jurisdiction necessarily in the public domain elsewhere.